For some time, it has been known to utilize hydraulic actuators connected to the linkage of an aircraft control stick to provide force to the control stick which is dependent in some fashion upon the position of the stick and other aircraft parameters, to indicate to the pilot the degree of command caused by him, which in turn is an indication of the loading of the aircraft surfaces.
In such systems, the position of the stick and other parameters are monitored with suitable transducers and a corresponding force command is generated. As the pilot moves the stick, the force changes commensurately. Such systems also generally have a trim position, which is equivalent to the old style detent wherein the force is a null at a selected position, giving the stick positional stability when in the trim position. Any change in the trim position changes the position/force relationship provided by the force command generator.
Because such force feel systems actually provide a force input to a stick, any erratic behavior thereof will provide actual commands to the control system of the aircraft, unless such force is overcome by the pilot or any automatic flight control systems. For this reason, open-loop force commands are favored only if they are implemented in sufficiently complex systems which can monitor any faulty operation and overcome it, while at the same time not impairing desired operation.
The typical hydraulic force feel system employs hydraulic servo actuators, the mechanical output of which is effective in either the forward or the reverse direction of stick motion directly on the mechanical linkage associated with the cyclic stick. The hydraulic servo actuator is controlled by an electrohydraulic servo valve which has two outputs, one relating to each of the directions of motion of the hydraulic servo actuator, the outputs having together a differential hydraulic pressure which is a function of the magnitude and polarity of a force command signal applied to the valve, the differential hydraulic pressure output determining the force (by the ratio of the area) created by the actuator. Systems of this general type are illustrated in commonly owned U.S. Pat. Nos. 3,733,039 and 3,719,336. One problem with this type of system is that a true null position (zero force for zero electrical signal input to the servo valve) is hard to maintain for long time periods over wide variations of temperature of the hydraulic fluid utilized in the servo valve and the hydraulic servo actuator. Further, amplifier drift and other factors can result in long term drift of the null. To overcome this problem, the system described in commonly owned U.S. Pat. No. 4,078,749 includes means to sense conditions in which the differential pressure should be at a null, such as during trim release with small stick motion, measuring the differential pressure across the actuator at such times, and providing a compensation bias to the system as a function of the differential pressure, which compensation is applied until the next time that a null should occur, when the compensation can be updated. This has the obvious drawback of being intermittent in keeping up-to-date on the offsets, since it does not operate continuously. In addition, this type system does not accommodate changes in hysteresis, bias, linearity and the like which occur at other than the null position.
In many servo systems, it is possible to provide closed-loop control working around a null command. For instance, in a position servo, it is possible to command a certain position, and when that position is reached as indicated by feedback signals, the command is reduced to zero. Any variation in the command results in a command error signal for repositioning the device. In such devices, the use of proportional, integral and other gains in the servo loop is relatively straightforward. However, in hydraulic force feel actuators of the type described herein, a nulling servo loop is not generally possible because the utilization of the pressure control servo valve is preferred for variety of design reasons. In such systems, the maintenance of a given pressure by the force actuator is achievable only by maintaining a differential pressure across its inputs, which in turn requires maintaining a continuous pressure command (for the desired force) at the input of the pressure control servo valve. Therefore, nulling-type servo principles cannot be employed.